DNA replication

DNA replication copies a DNA molecule so genetic information can be passed to daughter cells. Complementary base pairing allows each parental strand to guide synthesis of a new strand. Replication is described as semiconservative: each daughter double helix contains one parental strand and one newly synthesized strand. In 1958, Matthew Meselson and Franklin Stahl reported density-labeling experiments in Escherichia coli whose results supported this model and distinguished it from conservative and dispersive alternatives.

Replication begins at DNA sequences called origins. Proteins open the double helix, creating replication forks where the strands are copied; the details and number of origins differ among organisms. DNA polymerases extend a growing strand by adding nucleotides complementary to its template, and can synthesize DNA only in the 5′-to-3′ direction. Since the two template strands run antiparallel, one new strand can be made continuously toward a fork (the leading strand), while the other is made discontinuously in short Okazaki fragments (the lagging strand).

Replication is carried out by coordinated proteins rather than by DNA polymerase alone. Helicases separate the strands, primases make short primers that provide a starting point for DNA synthesis, and DNA polymerases extend the new strands. Other proteins stabilize unwound DNA and relieve twisting strain. On the lagging strand, primers are removed or replaced and DNA ligase seals remaining breaks between fragments. The exact enzymes and their roles vary across bacteria, archaea, and eukaryotes.

Accurate copying is essential for cell growth and inheritance. Complementary base pairing, proofreading by many replicative polymerases, and additional cellular repair pathways help limit copying errors, although mutations can still occur. In eukaryotic cells, nuclear DNA is generally replicated during the S phase of the cell cycle; the completed copies are then distributed during cell division.